President Obama won’t be on the ballot in 2020, but he is meeting with some of the Democrats who are looking to run for president, giving them advice on how to challenge President Trump, according to news reports.

Mr. Obama has met with nearly a dozen potential candidates, coaching them on issues, party politics and how to confront the unorthodox Mr. Trump, CNN reported.

It’s a unique role for a former president who’s seen Mr. Trump, in less than two years, wipe away much of his legacy, from environmental policy to immigration decisions to trade agreements to the Iran nuclear deal.

Republicans doubt Mr. Obama’s advice will amount to very much, saying that even in office, when Mr. Obama wasn’t on the ballot his party got pasted in elections.

“There is no leader of the Democratic Party to this day and Michelle and Barack Obama have not done anything during their tenure in the White House to build on the Democratic brand,” said one GOP operative.

Mr. Obama left office promising to mentor Democrats, saying he wanted to build a more durable party. His chief effort has been trying to get a better outcome out of the decennial redistricting process, which draws the lines for U.S. House districts.

But with the 2020 elections looming, a number of his lieutenants, including Mr. Biden and Mr. Holder, are looking at presidential runs. The Democratic field is shaping up to be large.

Antjuan Seawright, a Democratic strategist and CEO of Blueprint Strategy, views Mr. Obama as a great unifier whose voice is needed ahead of the 2018 and 2020 elections, but he also thinks there’s a place for 2016 nominee Hillary Clinton to lend a helping hand to future 2020 candidates.

“You either win or you learn,” Mr. Seawright said. “We can learn a lot from her race.”

Mr. Obama may also be able to help Democrats connect with black voters, and his advice on wooing donors is likely to be heeded, given his prowess as a master fundraiser.

“That is his biggest influence in the election … there is still a lot of the donor class who idolize him,” said Chuck Rocha, a political consultant who advised Sen. Bernie Sanders’s 2016 presidential campaign. “People are going to write checks and show up.”

But Republicans questioned what specific advice Mr. Obama can offer when it comes to facing Mr. Trump, who ran — and won — on a platform of erasing Mr. Obama’s legacy.

“The problem for the Democrats is their electoral strategy is, ‘lets get rid of Trump‘ … what they don’t understand is we wanted Trump to erase the legacy,” said Ryan Rhodes, who worked for Dr. Ben Carson’s 2016 presidential campaign.

Matt Beynon, who worked for Rick Santorum’s 2016 campaign, said Mr. Obama will be good at coaching the Democratic candidates ahead of their primary on how to maintain the Ivy League, affluent voters he was able to win in 2008 and 2012.

“It’s going to be fought over in places like Potomac, Maryland, and Greenwich, Connecticut — not the farm fields of Iowa,” he said.